Ball bonding is a common technique for interconnecting the bond pads on a semiconductor die with the contact points on a lead frame or other substrate on which the die is mounted. Electrical interconnect wires typically are run from the bond pads on the top of the die to lead fingers on a lead frame in order to electrically connect the circuitry on the die to the pins of the lead frame that will extend from the package after the die has been encapsulated. The wire bonds between the bond pads of the die and the lead fingers commonly are formed using a ball bonding machine. FIGS. 1A-1I demonstrate the steps in a conventional technique of ball bonding. The conventional looping technique (herein termed forward looping) involves ball bonding one end of a gold wire to a bond pad on a die and stitch bonding the other end of the wire to the lead frame. More particularly, using a ball bonding machine, the wire 17 is passed through a set of clamps 18 and through a center bore of a capillary 11. At the beginning of the process, a wire “tail” 23 is protruding from the tip of the capillary 11, as shown in FIG. 1A. The tail 23 at the end of the wire 17 is heated by means of an electric spark 16 termed an electric flame off (EFO) from an EFO wand 24. The spark melts the end of the wire, which, in turn, forms into a ball 19 when melted, as shown in FIG. 1B. The clamps 18 are closed during EFO in order to provide a current return path through the clamps and then are opened to allow the ball to seat itself in the capillary tip. The capillary 11 is then moved to a position above the bond pad 13 of the die 15, as shown in FIG. 1C.
The capillary 11 is then moved downwardly with the clamps 18 still open during the initial acceleration of the capillary and then are closed during deceleration of the capillary so that the ball remains seated during the downward motion of the capillary. The clamps then open just before the ball contacts the bond pad 13. The ball 19 comes into contact with the bond pad 13 on the die 15 with the clamps 18 still open, as shown in FIG. 1D. Heat and/or ultrasonic energy are applied to the die to cause the ball to become bonded to the bond pad 13. This bond typically is termed a ball bond or first bond. The capillary 11 is then raised with the clamps 18 opened to pay out a short length of wire that is still attached to the top of the ball bond, as shown in FIG. 1E. Next, with the clamps 18 open, the capillary 11 is moved through a predetermined looping motion with the wire (which is still connected to the ball bond) and trailing out of the capillary 11 to a position generally near and above the lead finger 21. With the capillary 11 positioned above the lead finger 21, the clamps 18 are closed, as shown in FIG. 1F. The capillary 11 is then lowered to pinch the wire between the capillary and the surface of the lead finger 21, as shown in FIG. 1G. Again, heat and/or ultrasonic energy may be applied to bond the pinched portion of the wire to the lead finger 21. This bond is termed a stitch bond or second bond. The clamps 18 are now opened again and the capillary 11 is then raised with the wire still attached to the stitch bond such that an additional wire “tail” 23 pays out of the capillary, as illustrated in FIG. 1H. The clamps 18 are then closed and the capillary 11 is raised further to snap the wire tail 23 at the weakest point, which is at the stitch bond location. The completed connection 22 is termed a wire loop and is illustrated in FIG. 1l.
At this point, the capillary is moved near the next bond pad on the die 15 for commencing the wire looping process for the next bond pad on the die. The wire tail 23 that remains protruding from the tip of the capillary after the conclusion of the formation of the preceding wire loop will be melted by EFO, as previously described, to form the next ball for commencing the next ball bonding operation. The above-described conventional forward ball bonding technique is fast, reliable, and inexpensive. However, it has limitations. Most notably, the minimal loop height is normally over 150 microns. Loop height is defined as the maximum height of the wire above the bonding surface, e.g., the top surface of the bond pad. Attempting to achieve lower loop height can cause neck damage to the wire loop. The neck is the portion of the wire loop directly adjacent to the ball bond. Reducing the loop height below 150 microns tends to weaken or break the neck.
There is an increasing demand for smaller and smaller integrated circuit packaging. One of the significant aspects of reducing the size of the integrated chip packaging is reducing its thickness or height. The thinner packages are generally referred to in the trade as low profile packages. Commensurate with the desire to reduce the height of the package is the desire to reduce the height of the highest point of the wire loops, which, in many instances, is the limiting factor as to the height of an integrated circuit package.
In order to reduce loop heights for integrated circuit packaging and other purposes, a wire looping technique known as reverse looping was developed. The premise behind reverse looping is that, because the highest point of the wire loop is adjacent the ball bond, it would be desirable reverse the looping process so as to make the first, ball bond on the lead frame (or other substrate) and make the second, stitch bond on the bond pad of the die because the surface of the lead frame is lower than the surface of the die. Hence, the highest point of the wire loop is near the lower bonding surface, thus reducing the overall height.
However, simply reversing the direction of the looping process would not be possible because, the stitch bond requires the capillary to come in contact with the bonding surface. The bond pads on a die usually are very small and, thus, it is difficult to make a stitch bond on a bond pad on a die without the capillary contacting and, hence, damaging surrounding circuitry on the die. Furthermore, the wire loops tends to sag to their lowest points close to the stitch bond. Thus, if the stitch bond site is higher than the ball bond site, the wire might contact the edge or the top surface of the die. This could lead to electrical shorts or breakage of the wire.
Thus, a reverse looping technique was developed, such as illustrated in FIGS. 2A through 2C, in which the first step is to form a ball bond 25 on top of the bond pad 27 on the die 29 essentially in accordance with standard techniques for forming ball bonds. However, instead of paying out the wire 17 as would be the case after making the ball bond in a conventional forward looping technique, the capillary 11 is raised, the clamps 18 are closed, and the capillary is raised further to snap the wire off from the ball bond leaving just the ball bond (or bump) 25 on the bond pad 27, as illustrated in FIG. 2A. Then a complete wire looping process is performed in the reverse direction, i.e., from the substrate to the bond pad. That is, a second ball bond 37 is then formed on the lead frame 39, the capillary 11 is then moved through a series of motions to a position above the first ball bond 25 to create the desired wire loop shape, as illustrated in FIG. 2B. Then, a bond 43 is formed on top of the first ball bond (or bump) 25. The completed wire loop is illustrated in FIG. 2C.
This reverse looping process can provide low loop heights for low profile packaging. However, it is a much slower process than forward looping because it requires the formation of two ball bonds per loop. Furthermore, the die must suffer greater impact because the capillary must form a bond on the die twice per wire loop (i.e., once to create the first ball bond and a second time to create the bond on top of the ball bond). Another limitation of reverse looping is that it often is the limiting factor on how fine the pitch of the bond pads on the die. Particularly, the bump 25 on top of the die bond pad must be large enough to provide support for a bond. In addition, the diameter of the bump will increase in the lateral direction when the bond is made on top of it.
Accordingly, it is an object of the present invention to provide an improved wire loop formation method and apparatus.
It is another object of the present invention to provide a wire loop interconnect with very low loop height.